Air Consumption Issues

[BradMM]

Yes, that's my certification level but not my occupation. I thought this was a place to learn more... guess I was wrong.

Nitrogen uptake has nothing to do with how much exposure your body gets to whatever gas your breathing... it's always the same, right? There went the benefits of diving with Nitrox!

 

[BradMM]

Now THIS was helpful!

Air consumption - nitrogen absorption

 

[Blasto]

If my SAC rate is higher, then I'll be taking in more N which would cause the tables to be less useful for me than someone who fits the profile of "average."

Nitrogen uptake has nothing to do with how much exposure your body gets to whatever gas your breathing... it's always the same, right? There went the benefits of diving with Nitrox!

Nitrogen uptake has almost nothing to do with your breathing and SAC.

Your body is equalized to ambient pressure immediately and you take up the extra nitrogen at the beginning of your first breath. So if you breathe at all, the PPN2 and PPO2 you get are determined by your depth and your gas mixture.

So N2 intake isn't always the same - it's proportional to %N2*pressure. For EAN36, it's about 20% less than for air.

The only time SAC becomes a deciding factor is if it's 0, as in freediving, since the amount of additional nitrogen to saturate the tissues is hard-limited by what you bring. Even then, there have been (a lot of) DCS cases in freedivers, so apparently even the small amount of N2 already in the body and the lungs can be sufficient, under some conditions, to cause it.

 

[scubadada]

Yes, that's my certification level but not my occupation. I thought this was a place to learn more... guess I was wrong.

Nitrogen uptake has nothing to do with how much exposure your body gets to whatever gas your breathing... it's always the same, right? There went the benefits of diving with Nitrox!

You have that totally wrong, it has everything to do with your exposure to the partial pressure of nitrogen. It is truly frightening that you are a divemaster

 

[RayfromTX]

Blasto- I am quoting portions of the discussion that was linked to earlier in this thread. I recommend the thread. It is a good discussion. What do you think about the points that Cameron raises about rates of perfussion being tied to rates of metabolism which is tied to rates of breathing. It makes a case for not being able to make blanket statements about nitrogen uptake and rate of breathing.

At the blood/lung barrier, this might be the case, but what about the rest of the body, which is really where on-gassing and off-gassing matters? If you are assuming a diver is a big blob of gel, it might not matter, but physiology and the laws of thermodynamics tie breathing rate to metabolism. Perfusion changes with changes in metabolism, as does pressure gradients at various tissues in the body. In other words, changes in breathing rate indicate changes in metabolism indicate changes in perfusion indicate changes in rates of on-gassing/off-gassing.

Cameron

While there is a voluntary portion to breathing rate, this is not something that is without cost. In other words, while you might be able to consciously slow or increase your breathing rate separately from changes in your metabolism, you cannot sustain this difference very long without causing problems.

In this example, if one person is breathing 1.5 times faster than the other for any time significant to decompression, for whatever reason, underlying that is a higher metabolism with a higher rate of gas exchange. Otherwise, either one person is hypervenilating, reducing blood CO2 levels below that which is necessary, and will eventually pass out, or the other person is hypoventilating, allowing CO2 levels to rise above what is sustainable, and will eventually pass out. Interestingly enough, the end result is the same because this is the only way the body can forcibly take over breathing rate and match it back to metabolic need.

Even differences in resting breathing rate are directly attributed to differences in resting metabolism- for a healthy person, these are practically synonymous. Our metabolisms are largely aerobic, meaning that they are fueled by a partial pressure differential of oxygen, and regulated by a partial pressure differential of CO2. This is at the blood tissue barrier, which can be a little harder to conceptualize than just thinking about the partial pressure of inspired gasses. If one person has a higher resting breathing rate than another, they have a higher resting metabolism and hence, a higher resting rate of gas exchange.

Complicating the answer even futher, an increased metabolism also allows faster off-gassing, so there is somewhat of a wash here when we think of the metabolic contribution to decompression obligation. Really, though, we're not interested in counting molecules- we're only concerned with managing tissue partial pressures relative to ambient pressures. It comes down to capacity versus rate of input versus rate of output. This is where the ability to think in terms of calculus really helps, but how many of us have ever really been able to do that. <g>

So, I can see why the rote answer to the question "does breathing rate matter?" has been "no," but that is not really true. Breathing rate will not affect what the dive table tells you to do, but it does represent the underlying rates of gas exchange occuring in a diver's body, resting or active, and may indicate how much of the safety factor built into the tables you are taking advantage of.

Cameron

 

[Blasto]

It sounds like a valid concern, and something that is worth considering in research. But, then, bubble models sounded very good, like the next logical step, and then the NEDU study showed that the old simple Buhlmann model produced better results.
It also agrees with the established knowledge that more work correlates positively to more RMV and to higher DCS susceptibility, but it's not clear whether RMV is the link between them or only another consequence.

What the cases of DCS in freedivers confirm though is that if an identifiable relationship exists, it's definitely not linear. It's not clear enough yet (at least not to me) what it is and how to apply it. For practical reasons, I'd limit its application to setting conservatism a little higher or lower within the accepted range of values for known models. And, out of the same conservatism, mostly to being more conservative (lower GF) for divers with higher than usual SAC for their body mass.

 

[RayfromTX]

Most discussions I've seen follow along with the blanket statement that it is simply determined by the PPN2 and the time. I'd like to read more about the other factors and if the role they play is greater than negligible. The discussion of that will probably lead to me understanding the whole concept more fully. My suspicion is that people already compensate for increased activity or exposure to low temperatures by padding their shallow stops but I'd really like to learn some research results as well as the practices and experiences of the more advanced divers. I'd also like to find out how it is trained.